Matthew Stuart, P.E., S.E., F.ASCE

Course Outline

This one hour online course will provide the user with an understanding of the special design considerations necessary when dealing with unusual vertical and horizontal loads on steel joists and joist girders. This course includes a multiple choice quiz at the end.

Learning Objective

At the conclusion of this course, the student will:

• Understand the options available when designing joists for special uniform and concentrated vertical loads.
• Understand the capacity limitations of both joists and joist girders to resist lateral loads.
• Understand the analysis and strengthening of standard joist and joist girder bearing assemblies for the transfer of out-of-plane lateral loads.

Course Introduction

Often in the course of the design of a joist, loads in addition to uniform vertical gravity loads are imposed on the member. In such a situation, for an engineer to be able to select the appropriate member size or specify the correct load resisting performance of a joist, the designer must be able to properly interpret the manufacturer's allowable load tables and member capacities for both shear and moment. This course will guide the engineer through that process.

The design of the overall stability of a building involves an understanding of the load-path mechanism that distributes the imposed wind or seismic loads from the cladding to the foundations. Steel joists and joist girders are limited in this regard. This course will help guide the engineer through the understanding of what these limitations are.

Course Content

VERTICAL LOADS

Joist manufacturers recommend that the most economical option (from a material quantity point of view) for the support of concentrated or non-uniform loads is to designate the use of special joists through the use of specific load diagrams. In addition to providing a load diagram to the manufacturer, it is also recommended that the design engineer verify that the end seat of the special joist is compatible with the end seat depth of the adjacent joist series. In general, the maximum shear capacity of a 2.5 inch deep end seat is limited to about 9.2 kips. End reactions greater than this will require that the special joist be supplied with a deeper seat. Alternatives to submitting load diagrams to the manufacturer include:

1. Using a heavier standard joist that is capable of supporting a equivalent uniform load that envelops the shear and moment diagram for the actual loads imposed
2. Specifying a KCS series joist that provides a shear and moment envelope that is greater than that imposed by the actual loads
3. Substitute a wide flange beam capable of resisting the imposed loads

Load diagrams or plans for net uplift loads should always be provided because only the joist manufacturer is capable of checking members for the stress reversals associated with these types of vertical loads. The following is an example of a multi-purpose loading diagram for joists.

In specifying standard joist sizes as an alternate to special joists, the following precautions should be taken. The actual uniform load calculated from the load diagram must fall completely within the equivalent shear diagram of the standard joist selected. The allowable shear diagram can be constructed from information derived from the joist load tables and Steel Joist Institute (SJI) specifications. The maximum allowable end shear is equal to the allowable uniform load times half the span for a given joist. The allowable shear at the center of the joist is a percentage of this end shear value. The percentage is given in the SJI specifications for all joist series available. K series joists are designed for centerline shear of 25% of the maximum end shear. The point of zero shear for the special load diagram should also be determined. If this point is not relatively close (one foot +/-) to the center of the joist span, there may be diagonal members that are subject to stress reversal. If stress reversal is present with the use of a standard joist, a KCS series joist must be specified only as an alternate to the special joist.

Collateral hanger loads that do not exceed 150# can be accounted for as uniform loading. When hanger loads exceed 150 pounds, they should be accounted for with special load diagrams using concentrated loads. An exception to this rule that commonly occurs includes the support of sprinkler lines, cable trays and ducts. For these items, it is common for hanger loads to exceed 150#. Joist manufacturers indicate that for these type of hanger loads, the use of uniform loads to account for the actual loadings has proved to be reasonable and economical. However, it is recommended that the design engineer specify on the drawings the hanger spacings that are required in order to help minimize the magnitude of the reactions. In addition for these type of hangers, web reinforcement should be employed for hangers that occur at locations greater than 6" from the bottom chord panel points.

Typically the method for determining the proper distribution of loads on a series of adjacent joists is based on a rigid support transfer element resulting from static equilibrium, however, there are situations where the elastic nature of the supports and joists results in a different distribution of the support reactions. The relative stiffness of a series of joist based on a given distribution beam, RSJ = ((K/S/(4EI))1/4. Where; K = the individual joist stiffness (kips/inch); S = the joist spacing; E = the beam modulus; I = the moment of inertia of the beam. If S < (3.14/4 RSJ) the beam on elastic support calculations are applicable. If the spacing limit is not exceeded and the length of the beam is less than 1/RSJ the beam may be considered to be rigid with respect to the supporting joists and the reactions to the joists may be determined by static equilibrium. In lieu of using spreader beam below or above the joist it is also possible to field fabricate a truss within and between the joists to distribute the load more effectively between several members.

Joist manufacturers recommend that for eccentrically loaded joist girders or beams, the joist spacing be staggered on each side of the girder by at least 6" to allow for the bearing seats of the joists to extend past the center line of the girder. This offsetting helps to compensate for the twisting action induced on the girder due to differences in joists reactions from opposing sides of the girder. Extreme situations may actually require the addition of bottom chord extension bracing from the supported joists to limit the twisting action.

LATERAL STABILITY & LOADING

The minimum deck fastening requirements as required by SJI specifications for adequate joist top chord stability is 100 PLF (300# at 3'-0" o.c.) For K-series joist and from 120 PLF to 250 PLF (depending on the chord size) for LH and DLH joists.

The capacity of joists and joist girders to function as chord members and collector elements for diaphragm loadings is discussed extensively in “Designing With Steel Joists, Joist Girders, Steel Deck” by Fisher, West & Van De Pas. This manual was published by Nucor Corporation, however it is no longer in publication. Your local joist manufacturer or steel fabricator may be able to provide you with a used copy. The following pertinent items are worth noting from this same referenced text. The maximum chord force permitted by K-series joists is 6.8 kips (see pages 230 and 231).

The maximum chord force permitted by joist girders weighing more than 30 PLF is 20 kips (see Table 7.1.1, page 228).

Chord forces greater than these values can be obtained through extensive modification to the standard chord/bearing assembly of each section, however, most joist manufacturers recommend the use of wide flange spandrel beams in lieu of joists for bracing/chord forces greater than 20 kips. Joist girders can be practically modified to provide chord force limits of up to 200 kips (see Detail F, pages 93).

For situations in which the perimeter edge roof angle, acting as a diaphragm collector, is perpendicular to the span of the joists, the magnitude of the collector force should be compared to the rollover capacity of the joist seat. The rollover capacity of a typical is about 1650#. For a typical joist spacing of 6'-0" o.c., this capacity would equate to a maximum collector force of 275 PLF. Tests performed by Vulcraft indicate the actual upper bound of the ultimate rollover strength limit of some H-series joists was 9.0 kips. Examples of rollover analysis can be found in Section 7.6, pages 268 through 273.

In either case, the designer should determine if the magnitude of the collector force in this situation warrants the use of “drag strut” shear transfer devises such as that shown in Figures 4.2.8 and 4.2.9, page 70.

Plan, laminated wood and structural wood fiber decks are not normally used as diaphragms as there has not been adequate research performed by the industry. Standing seam roofs are also not capable of functioning as an adequate diaphragm. In all of the above cases, alternate in-plane horizontal bracing must be provided as a part of the roof system.

Course Summary

The designer must compensate for non-uniform loads on joists by developing the shear and moment diagram for a particular member in order to establish an equivalent uniform load for use in selecting a standard joist or specify a KCS series joist. A third alternative involves the development of a loading diagram to enable the joist manufacturer to design and fabricate a special joist for each load case.

Lateral diaphragm collector and chord forces must be transmitted between the metal deck and the lateral resisting system via the joist or joist girder member chords. Standard joists and joist girders have limited capacities available to accomplish this goal. Special detailing provisions may be required when the member bearing seat assembly is not capable of transferring the magnitude lateral loads involved.

Additional Resource

You can download a program from Vulcraft's website called Vulcraft Assistant that helps you design joists with concentrated loads (http://www.vulcraft-in.com/). The program does not do any deflection calculations.

Related Links

For additional technical informaion related to this subject, please visit the following websites or web pages:

Design/Build Speeds Construction of New Cold Storage Facility
Steel Joist Institute
Vulcraft
Ten Situations to Avoid when Designing with Steel Joists
Steel Solutions Ccenter - Joist the Facts
Up on the rooftop: How to design and detail non-uniform loads on steel joists

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.